The development of integration and miniaturization of semiconductor devices has been accelerated over years, and patterning techniques for a line width on the order of 45 nm are currently being brought into practical use. For such fine patterning, it is known that improvements of conventional excimer exposure technologies, that is, exposure techniques such as ArF immersion or double exposure can be applied.
It is known, however, that forthcoming patterning in the next generation for a finer line width of 32 nm or smaller may no longer be feasible by means of an exposure technique with an excimer laser, and an EUV exposure technique is considered to be promising, which uses extreme ultraviolet (EUV) light with a main wavelength of 13.5 nm that is shorter than that of an excimer laser.
To bring the EUV exposure technique into practical use, it is necessary to solve technical problems with each of element technologies, such as a dust protective pellicle, for preventing contamination from being attached to a light source, a photoresist, and a photomask. Among these element technologies, there has already been a significant development in those for a light source and a photoresist. However, various problems still remain to be solved for the dust protective pellicle, which affects manufacturing yield of semiconductor devices and the like, and constitute a barrier to the practical use of the EUV exposure technique.
Specifically, unsolved technical problems with a pellicle for use in the EUV exposure include: (1) development of a material that is highly transmissive to EUV light and is chemically stable; (2) an arrangement for holding the transmissive film (pellicle film), which is necessarily a ultrathin film, under constant tension without slack; and (3) capability of being used under vacuum after the film is bonded to a photomask under normal pressure. Among these unsolved technical problems, the problem of the above item (1) is particularly serious, and in practice, material development for a transmissive film that has high transmittance to EUV light and is chemically stable without being changed due to oxidation or the like are still far from realization.
Materials that have been used for conventional pellicle films (mainly organic materials) are not transparent to the wavelength band of EUV light, and therefore they are not transmissive to EUV light. In addition, the materials have a problem of decomposition or degradation due to radiation of light. To date, no material that is fully transparent to the wavelength band of EUV light has been known; as a relatively highly transparent material, however, silicon has drawn attention and has been documented.
For example, see Shroff et al. “EUV pellicle Development for Mask Defect Control,” Emerging Lithographic Technologies X, Proc of SPIE Vol. 6151 615104-1 (2006) (Document 1), and Livinson et al., U.S. Pat. No. 6,623,893 B1, “PELLICLE FOR USE IN EUV LITHOGRAPHY AND METHOD OF MAKING SUCH A PELLICLE” (Document 2).
The silicon used in a pellicle for EUV exposure reported in the Document 1, however, is a silicon film deposited by a method such as sputtering. Since such a film is necessarily amorphous, the film has high absorptivity (absorption coefficient) to light in the EUV wavelength band.
Similarly, the silicon used in a pellicle for EUV exposure reported in the Document 2 is as a premise a film deposited by a method such as CVD. In this case, since the silicon film is also amorphous or polycrystalline, the film has high absorptivity (absorption coefficient) to light in the EUV wavelength band.
Additionally, a modest tensile stress is desirable on a silicon film bonded to a frame as a pellicle film. However, since an excessive stress may cause a damage, it is desirable that, during bonding of the silicon film, the temperature should be at or a little more than the room temperature. However, in the conventional silicon films as described above, a strong stress is already introduced during deposition, such as sputtering and CVD processes.
Since these silicon films are not a single crystal silicon film, amorphous portions or grain boundaries included in the film cause absorptivity (absorption coefficient) to EUV light to increase, and therefore transmittance thereto to decrease. Moreover, these silicon films are chemically unstable and easily oxidizable. Therefore, these films are not practical because the transmittance to EUV light decreases over time.